Correspondence Address:
Fumi MizuhashiDepartment of Removable Prosthodontics, The Nippon Dental University School of Life Dentistry at Niigata, 1-8 Hamaura-Cho, Chuo-Ku, Niigata, 951-8580 Japan

Abstract

Background/Aim: The aim of this study was to investigate the influence of root length over alveolar bone to the occurrence of root fracture using cone-beam computed tomography (CBCT). Methods: We reviewed the CBCT images of 176 endodontically treated upper incisors (7 with root fracture and 169 without root fracture) from April 2018 to March 2019. Measurement of the mesial and distal root length over and under the alveolar bone was performed by cross-sectional multiplanar reformation (MPR) image that passing the middle of the distance from the labial side to the palatal side of the tooth, and measurement of the labial and palatal root length over and under the alveolar bone was performed by parasagittal MPR image that passing the middle of the distance from the mesial side to the distal side of the tooth using CBCT. The minimum and average values among the labial, palatal, mesial, and distal root length over and under the alveolar bone were evaluated. The minimum and average values of the root length ratio of over the alveolar bone to under the alveolar bone were also evaluated. Statistical analysis was performed by Mann–Whitney U-test. Results: The minimum and average values of root length over the alveolar bone were statistically significantly different between the tooth with and without root fracture (P < 0.01). The minimum value of the root length ratio of over the alveolar bone to under the alveolar bone was statistically significantly different between the tooth with and without root fracture (P < 0.001). Conclusions: These results suggested that the root length over the alveolar bone was smaller in the tooth with root fracture.

The number of dental patients with caries or periodontal disease has been decreased by the development of preventive dentistry, and the cause of tooth loss has also been changed. The incidence of tooth loss caused by root fractures is increasing, whereas that of caries or periodontal disease is decreasing.[1] It is desirable to prevent root fractures for avoiding tooth loss.

Remaining root tissue is necessary for increasing the fracture resistance. Ng et al.[2] indicated that the fracture susceptibility of teeth restored with posts may be related to the amount of the remaining tooth structure, which provides resistance to the fracture of the tooth. The minimum thickness of root dentine required around a post is uncertain, and values from 1.0 to 1.75 mm are often suggested.[3] Juloski et al.[4] showed that the presence of a ferrule increases the fracture resistance. Biologic width is the width including the connective tissue attachment and the junctional epithelium, and the mean dimension is approximately 2 mm.[5] In cases where caries reaches the subgingival part, the margin of the crown tends to set at deep part under the gingival margin, and then the biologic width is infringed. When the biologic width is infringed, the amount of the residual dentine becomes smaller and there is a susceptibility to cause root fracture.[6] Furthermore, it is difficult to give ferrule which can prevent vertical and horizontal root fractures [7],[8] on teeth with infringed biologic width. Therefore, when the distance between the margins of the crown and the top of the alveolar bone was <2 mm, crown lengthening or orthodontic extrusion is desired to be performed for obtaining the biologic width and ferrule in order to prevent inflammation of the gingiva or root fracture.[9],[10] However, the relationship between the root length over the alveolar bone and the occurrence of root fracture has not been investigated clearly.

Intraoral radiography has been used commonly for the examination of tooth and periodontium; however, the amount of remaining tooth structure can be detected only in the two-dimensional image. In contrast, the amount of remaining tooth structure can be observed from all directions using cone-beam computed tomography (CBCT). Recently, CBCT is used for the detection of root fracture, which provides more information about dental structure.[11]

The aim of this study was to investigate the influence of root length over the alveolar bone to the occurrence of root fracture on endodontically treated teeth using CBCT.

CBCT imaging was performed with a CBCT unit (Fine Cube; Yoshida, Tokyo, Japan). The CBCT parameters were as follows: tube voltage, 90.00 kV; tube current, 4.00 mA; field of view, 81 × 81 mm; and rotation time, 16.8 s. The protocol was set at a thickness of 0.144 mm, resulting in axial, cross-sectional, and parasagittal multiplanar reformation (MPR) images and three-dimensional images.[12]

The root length over the alveolar bone (between the margins of the crown and the top of the alveolar bone) and under the alveolar bone (between the top of the alveolar bone and the root apex) was measured automatically by drawing a line on CBCT. The root length over and under the alveolar bone was measured at four points such as mesial, distal, labial, and palatal parts. The measurement of the mesial and distal root length over and under the alveolar bone was performed by a cross-sectional MPR image that passes between the middle of the labial and palatal margins of the crown and orthogonal to the middle of the mesial and distal margins of the crown [Figure 1]a. The measurement of the labial and palatal root length over and under the alveolar bone was performed by parasagittal MPR image that passes between the middle of the mesial and distal margins of the crown and orthogonal to the middle of the labial and palatal margins of the crown [Figure 1]b.{Figure 1}

The minimum and average values among the labial, palatal, mesial, and distal root length over the alveolar bone of each tooth were calculated. The minimum and average values among the labial, palatal, mesial, and distal root length under the alveolar bone of each tooth were also calculated. In addition, the minimum and average values of the root length ratio of over the alveolar bone to under the alveolar bone among the labial, palatal, mesial, and distal root length of each tooth were calculated.

The sex difference between patients with and without root fracture was analyzed by cross-tabulation. The age difference between patients with and without root fracture was analyzed by Mann–Whitney U-test. The differences of the minimum and average values of the root length over and under the alveolar bone between the tooth with and without root fracture were analyzed by Mann–Whitney U-test. The differences of the minimum and average values of the root length ratio of over the alveolar bone to under the alveolar bone between the tooth with and without root fracture were also analyzed by Mann–Whitney U-test. Statistical analysis was performed using statistical analysis software SPSS 17.0 (SPSS Japan Inc., Tokyo, Japan), and differences of α < 0.05 were considered significant.

Results

The result of Pearson's Chi-square test showed that there was no sex difference between patients with and without root fracture (χ2 = 0.457; P = 0.499). Age was not statistically significantly different between patients with and without root fracture (P = 0.620).

[Figure 2] shows CBCT images of the upper right central incisors with and without root fracture. The distal root length over the alveolar bone can be observed shorter on the tooth with root fracture [Figure 2]a in comparison with the tooth without root fracture [Figure 2]b.{Figure 2}

[Table 1] shows the results of the statistical analysis. The minimum and average values of root length over the alveolar bone were statistically significantly different between the tooth with and without root fracture (P < 0.001 and P < 0.01, respectively), and the root length over the alveolar bone on the tooth with root fracture was smaller than that without root fracture. The minimum and average values of root length under the alveolar bone were not different between the tooth with and without root fracture (P = 0.946 and P = 0.284, respectively). The minimum value of the root length ratio of over the alveolar bone to under the alveolar bone was statistically significantly different between the tooth with and without root fracture (P < 0.001), and the root length ratio of over the alveolar bone to under the alveolar bone of the tooth with root fracture was smaller than that without root fracture. The average value of the root length ratio of over the alveolar bone to under the alveolar bone was not statistically significantly different between the tooth with and without root fracture (P = 0.147).{Table 1}

Discussion

Preventive dentistry has been developed in recent times, and tooth loss caused by root fracture is increasing. This study investigated the influence of root length over the alveolar bone to the occurrence of root fracture on endodontically treated teeth using CBCT.

This study subjected the upper incisors to investigate the influence of root length over the alveolar bone on fracture resistance. The occurrence of root fracture on endodontically treated teeth is larger than that on vital teeth,[13],[14] therefore, only the endodontically treated teeth were subjected in this study. Traumatized teeth were removed from the study patients because the trauma can occur on any teeth in spite of the amount of the remaining tooth structure. Immature teeth and moderate or severe periodontal disease was eliminated from the study patients in order to integrate the condition of the study patients. This study investigated the teeth with crown prosthesis after endodontic treatment. There were no statistically significantly differences in the sex and age between patients with and without root fracture in this study. Concerning the core materials, Zhou and Wang [15] mentioned that several studies compared root fracture between fiber posts and cast posts; however, the results were inconsistent or conflicting. Therefore, the core materials were not taken into consideration in the analysis of this study. Concerning the post length, Büttel et al.[16] and Cecchin et al.[17] reported that post length has no effect on fracture resistance. Therefore, the post length was not taken into consideration in the analysis of this study. The factors that could influence the occurrence of root fracture were excluded when possible.

The results of this study showed that the minimum and average values of root length over the alveolar bone were statistically significantly smaller on tooth with root fracture in comparison with that without root fracture. This result was supported by the report that mentioned the importance of the amount of remaining tooth structure to provide resistance of the tooth.[2] It was reported that when force is applied to endodontically treated teeth, the occurrence of root fracture increases.[18] The lateral force applied to the crown part is passed around the dentine at the alveolar bone and tip of the post.[18] When the root length over the alveolar bone is small, larger force could be applied to the alveolar bone and tip of the post, and then, the root fracture will occur. The minimum value of the root length over the alveolar bone was 0.63 ± 0.20 mm on tooth with root fracture and 1.44 ± 0.46 mm on tooth without root fracture. Within the limitations of this study, it was suggested that the root length over the alveolar bone should be ≥1.0 mm for the prevention of root fracture.

Although the minimum value of the root length ratio of over the alveolar bone to under the alveolar bone of the tooth with root fracture was smaller than that without root fracture, there were no differences in the minimum value of the root length under the alveolar bone between the tooth with and without root fracture. These results suggested that the root length under the alveolar bone was not influenced by the root fracture, and only the root length over the alveolar bone was influenced by the occurrence of root fracture.

The results of this study suggested that the root length over the alveolar bone should be maintained for the prevention of root fracture. The limitation of this study was that the number of tooth with root fracture was small. In future studies, we should examine the influence of root length over the alveolar bone on fracture resistance in a large number of patients with root fracture.

Conclusions

This study investigated the influence of root length over the alveolar bone on fracture resistance of endodontically treated teeth using CBCT. The results suggested that root length over the alveolar bone is smaller at the tooth with root fracture in comparison with that without root fracture.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

References

1

Axelsson P, Nyström B, Lindhe J. The long-term effect of a plaque control program on tooth mortality, caries and periodontal disease in adults. Results after 30 years of maintenance. J Clin Periodontol 2004;31:749-57.